Animating and creating pose libraries without Shape Keys
I think the answer is simple : you may need up to one bone per vertex to obtain the same accuracy as shape keys.

About Karan Shah : he uses UV warp modifier which is purely based on the UV mapping. So that does not change the 3D (or 2D) geometry. Also, in this approach, you cannot mix the effect of two bones (and shape keys can be mixed).
If we want to compare shape keys effect on the face of a Manuel Lab character to UV warp effect for Karan Shah, you'll see that for each shape keys combination (for ML), you may need one texture part (for KS).

After that, maybe something new could be invented... I don't know.